Centrifugal cleaners have been employed for many years in the paper industry for removing small particles of higher specific gravity than paper fibers from slurries of paper making fiber, especially waste paper stocks. In centrifugal cleaners used for that purpose, in what is hereinafter referred to as "conventional centrifugal cleaning technique," the discharge outlet at the apex (tip) of the cylindrical-conical vessel is relatively small in comparison with the inlet and accepts outlets, e.g. 1/8 inch in diameter as compared with 5/8 inch diameters for the other two ports in a conventional cleaner 3 inches in diameter. In such conventional cleaning operations, therefore, the reject discharge through the apex outlet is correspondingly small in comparison with the accepts flow, e.g. 3% and 97% respectively.
In comparatively recent years, centrifugal cleaners have also been used to fractionate paper fiber slurries as well as to separate good paper fibers from contaminants of closely similar or lower specific gravities, by modifying the construction and/or operation of a conventional cleaner to provide operating conditions which cause the good fiber to be discharged through the apex outlet as the accepts flow while the lights are discharged as reject through the base (top) outlet which is the accepts outlet in conventional centrifugal cleaning. Such modified operation of centrifugal cleaners for use in fractionating mixtures of springwood and summerwood paper fibers is disclosed in Pesch U.S. Pat. No. 3,085,927 (1963) and Malm U.S. Pat. No. 3,352,745 (1967). Neither of those patents suggested any change in the flow areas of any of the ports of the conventional centrifugal cleaner referred to therein.
Grundelius et al U.S. Pat. No. 3,486,619 (1969) disclosed the use of a centrifugal cleaner for separating shives and other undesirable lightweight particles from a slurry of paper fiber by operation under controlled conditions causing the good fiber to be discharged through the apex outlet and the shives and other contaminants to be discharged through the normal accepts outlet in the base of the cleaner. Grundelius et al emphasized the importance for their purpose of maintaining the same through-flow volume which was normal for the cleaner when used for conventional cleaning purposes with respect to contaminants of higher specific gravity than paper fibers, and they suggested that the desired relative discharge flow conditions could be established and maintained by making the effective flow area of the apex outlet larger than that of the base outlet, and alternatively by means of throttling valves, preferably effective on the base outlet.
Braun U.S. Pat. No. 3,912,579 (1975) disclosed the use of a modified conventional centrifugal cleaner for the removal of certain contaminants indigenous to commercial waste paper, namely ink peppers, clays, pigment fillers and resinous materials used in coating, printing, laminating and binding. According to this patent, "reverse" centrifugal cleaning is effected in a conventional centrifugal cleaner modified by having its apex outlet enlarged to more nearly the same flow area as the base outlet, the specific diameters being 1/2 inch and 5/8 inch respectively for a 3-inch cleaner.
Braun disclosed that the through-flow could range from the same value used for "normal" centrifugal cleaning to considerably greater than normal, the specific examples being approximately 22 and 33.5 G.P.M. for a 3-inch cleaner, but the claims of this patent are limited to a through-flow substantially higher than normal and also to a flow through the apex outlet port at least as great as the flow through the base outlet port, the specific example being 55% accepts flow through the apex port and 45% rejects flow through the base port. In the description that follows, and also in the claims, the disclosure of the Braun patent with respect to standards of "normal" centrifugal cleaning is incorporated by reference, and in every instance where the term "normal" or "conventional" is used, it refers to such conventional, as distinguished from reverse, centrifugal cleaning and centrifugal cleaners.
Reference should also be made to the two patents issued to the assignee of the present application. Marsh et al U.S. Pat. No. 3,849,245 (1974) disclosed and claimed the use of reverse centrifugal cleaning for the purpose of eliminating grease from a slurry of paper fibers contaminated by garbage, the dimensions of the several ports in the centrifugal cleaner being of the same order as in the Braun patent. They also emphasize the use of low consistency for the stock to be cleaned, i.e. in the low end of 0.3-0.7% solids.
Raymond et al U.S. Pat. No. 3,849,246 (1974) emphasized the elimination of meltable contaminants, such particularly as grease and wax, from waste paper slurries by reverse centrifugal cleaning after heating the slurry to a temperature at which the contaminants were softened sufficiently to separate from the fibers to which they have a tendency to cling when cold. Otherwise, the disclosure of Raymond et al with respect to the dimensions and operating conditions of the reverse centrifugal cleaner closely corresponds to Marsh et al.
In summary, the state of the prior art of reverse centrifugal cleaning at the time the present invention was made was in effect that preferred operating conditions called for the use of 3-inch conventional centrifugal cleaners which had been modified by increasing the size of the apex (tip) outlet to approximately the same size as the inlet port and base (top) outlet, and also called for a higher pressure drop and through-flow rate than for normal centrifugal cleaning, with heating of the slurry being preferred when the contaminants to be eliminated were of meltable nature. The prior art patents also agreed that effective reverse centrifugal cleaning called for low consistency, namely 0.3 to 0.7% solids.